Two Harmonic Analyzers. 229 



half an inch in the ball, and that of another, 1,270, was 

 easily identified. 



The instrument already described can clearly only be 

 used on a structure while it is so disturbed as to set its 

 members vibrating. Such disturbances can generally be 

 set up by a shock of some sort, but when it is necessary to 

 cause artificial disturbance, it is better to adopt a periodic 

 disturbance of such varying period as will come gradually 

 into coincidence with the periods of free vibration, bringing 

 these vibrations out separately, when they will be readily 

 identified with the Analyzer, if not otherwise perceptible. 



For this purpose, in 1887, I adopted the following 

 method : — -A small cast-iron pulley, 6 inches in diameter, very 

 much out of balance, was mounted on a small frame that 

 could be clipped on to any part of the structure, and a cord 

 passed over this pulley on to a larger wheel, which was 

 turned by hand. In this way the unbalanced wheel was 

 driven at a gradually increasing rate until steady vibrations 

 in the structure were observed, then these coincided with the 

 period of the unbalanced wheel, and this was ascertained to 

 be about 1,200 by counting the revolutions of this hand- 

 wheel. At this speed the disturbing force resulting from 

 the unbalanced weight, 2 lbs. on a radius of 2 inches, would 

 be 40 lbs. The structure thus under examination was an 

 iron standard, very stiff. A theodolite was adjusted, with 

 the cross curves on a mark on the top of the standard, which, 

 when the period of the small unbalanced wheel coincided 

 with that of free vibration, was seen to move as much as one- 

 twentieth of an inch. Chains were then attached to the top 

 of the standard, and by means of blocks, a horizontal force 

 of a ton was thrown on to the top of the standard, when it did 

 not yield more than two-hundredths of an inch. So that 

 the deviation caused by the periodic force of 40 lbs., in such 

 coincidence with the period of free vibration as could be 

 attained with the hand-wheel, was three times as great as 

 that which resulted from a direct statical force of one ton. 



